The present invention relates generally to jet propulsion for aircraft, and more particularly, to a combined variable area nozzle and jet flow deflector for jet engines.
Modern aircraft are provided with jet propulsion means which usually consists of two or more jet engines disposed in engine nacelles mounted in or on the wings of the aircraft and having nozzles for directing the jet flow to the rear.
In many cases, the nozzle area or shape must be varied during flight to achieve superior performance under various flight conditions. This results from the combination of engine design factors and the differing requirements for low speed and high speed flight. Thus, a turbo fan engine with a relatively low design fan pressure ratio has a nozzle pressure ratio, with a confluent primary, ranging from approximately 1.3 at reduced power for low flight speeds to about 2.5 at full power for high subsonic flight speeds. If the nozzle has a relatively large convergence angle of the order of 20.degree. or more, the discharge coefficient undergoes a variation of the order of 10% over this range of pressure ratios, which requires a variation of the nozzle area of the order of 10% to maintain acceptable engine performance.
Changes in the nozzle configuration are also required for different flight conditions. Thus, for low speed flight, the lift must be increased. This can be accomplished, utilizing the Coanda flap principle, by blowing the jet flow over the upper surface of the wing and a continuous surface flap, if the ratio of the jet height to the flap radius is low. This is best achieved with a nozzle configuration of high aspect ratio, such as an oblate or D-shaped nozzle, rather than a circular nozzle. In addition, spreading of the jet flow is required to maximize the portion of the wing area involved and thus the lift, and to minimize the jet thickness at the flap knee to achieve efficient turning of the flow. These requirements, of course, conflict with the requirements for high speed flight in which spreading of the jet flow must be minimized to reduce drag effects, and the jet flow must be directed straight aft of the aircraft clear of the wing surface.
It will be apparent that any attempt to meet these conflicting requirements with a fixed nozzle geometry would necessarily result in severe and usually unacceptable penalties on performance in one or more of the various possible sets of conditions. It has been proposed to use deflectors associated with the upper nozzle surface to deflect the jet flow downward for low speed flight, but this results in an unacceptable decrease in the nozzle area precisely in the situation where the nozzle area must increase. Other variable geometry schemes have also been proposed, involving variable nozzle side openings and various types of variable and retractable baffles and deflectors, in an attempt to achieve acceptable performance in the various flight regimes involved.